The Norwalk virus and related 27nm caliciviruses are an important cause of epidemic viral gastroenteritis that occurs in family, school, group, institutional, or community-wide outbreaks affecting adults, school-aged children, family contacts, and some young children as well. Most information concerning the epidemiology of this group of viruses has been generated using diagnostic assays that employed either native Norwalk virus antigen present in or purified from human stools or, more recently, recombinant Norwalk VLPs to detect serologic responses. This year, large- scale epidemiologic studies were initiated using newly developed diagnostic assays that employed recombinant calicivirus capsid antigens that represent viruses serotypically distinct from Norwalk virus. The association of several gastroenteritis outbreaks with Hawaii virus or Toronto virus, each serotypically distinct, indicates that a better understanding of the epidemiology of caliciviruses will emerge from this approach.